The reference for this appears in RFC2616 section 5.1.2 (http://tools.ietf.org/html/rfc2616#section-5.1.2) as "REQUEST-URI".
It is clearly a HTTPD specification, but it has become a standard Apache HTTPD environment variable, which in turn has become a invaluable PHP environment variable.
I'm not entirely sure when and how "REQUEST-URI" became a standard, but it is certainly used in that way.
There is no reference to it in the Common Gateway Interface (CGI) Specification documentation on Environment Variables:
http://hoohoo.ncsa.illinois.edu/cgi/env.html
It is clearly a common issue when large/popular PHP software implements a function to tackle the issue:
http://api.drupal.org/api/function/request_uri/6
Details of the REQUEST_URI are found here:
http://php.net/reserved.variables.server
With this in mind, I am on the understanding that PHP should support it, regardless of whether the HTTPD passes it or not.
Unless there are any complaints I will add this as an environment compatibility.